Fun with a capital C

void test1(char *pkt, char *cmd, char *arg, ...)
{
 char *fmt;
 va_list ap;
 va_start(ap, arg);
 *pkt = START;
 *(pkt += vsprintf(cmd = ++pkt, fmt = cmd, ap)) = SEP;
 for (char *ptr = strchr(fmt, '%'); ptr++; ptr = strchr(ptr, '%'))
  (void)((*ptr == '%' && ptr++) || va_arg(ap, void *));
 *(pkt += vsprintf(arg = ++pkt, fmt = arg, ap)) = END;
 va_end(ap);
}